Increasing running trim

Discussion in 'Powerboats' started by Ben Land, Oct 22, 2019.

  1. Mr Efficiency
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    Mr Efficiency Senior Member

    Damned if I'd be grinding laminate off the bottom. If you thought that idea would work ( last foot), just add a foot to the back of the boat and use as much rocker as you like, enough to pull the stern down.
     
  2. pacblue
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    pacblue Junior Member

    1/8” barely gets past the non-structural skin coat, no adverse effects, just finish it off with a good epoxy barrier cost. Nothing to be afraid of.
     
  3. Mr Efficiency
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    Mr Efficiency Senior Member

    You did say as much as 1/2 inch, an eighth is barely perceptible, and won't make any appreciable difference unless you are talking break-neck speeds
     
  4. pacblue
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    pacblue Junior Member

    I said to go in 1/8” increments if you are uncomfortable in the approach, then it becomes a trial an error process. Not for aluminum plate construction.

    I have done it up to 1/2” on production frp boats, usually produced with a chopper gun, where there is a lot of extra glass at the transom.
     
  5. Barry
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    Barry Senior Member

    I would like to know the WHY of increasing the bow up angle, trim angle. I have looked a few times for information regarding the optimum angle for various trim angles that produce the
    most lift for the least drag. I remember that with our boats at 12 degrees deadrise, the graph showed an optimum trim angle of between 4-6 degrees and as the deadrise increased, the
    optimum trim angle increased as well. Perhaps this is why many deep V hulls appear to run quite high bow up.
    I will have another look to see if I can find this. I know that previous threads have not discussed this or stated that you want the trim angle at 4 - 6 degrees irrespective of trim angle for most lift-least drag but that is not the case
    So the question to the OP is this, WHY do you want an increased trim angle?
     
  6. pacblue
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    pacblue Junior Member

    Agree, if I had to pick a single number, 4 degrees would be my choice based on a number of boat tests, even up to 20 degrees deadrise.
     
  7. Barry
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    Barry Senior Member

    The point that I was making is that with increasing dead rise the optimum trim angle increases. Ie a 12 degree boat a 4 - 6 degrees was optimum and I remember
    this number because this was our dead rise on our river jet boats but the optimum trim angle went up with the increasing dead rise angle
     
  8. pacblue
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    pacblue Junior Member

    You can’t tell me that the optimum trim angle continuously increases with an increase in deadrise, are you saying that a 24 degree deadrise has an optimum trim of 9 - 12 degrees, for instance? And by optimum, what do you really mean? Were does it end, what’s the limit then?

    I have tested plenty of Sea Rays, Cobalts, Crownlines and Baja’s and 4 degrees from the static trim angle was the sweet spot for most cases.
     
  9. Barry
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    Barry Senior Member

    I am not
    I did not suggest that a 24 degree deadrise has an optimum trim of 9 - 12 degrees. I am suggesting you read Savitsky's paper "Hydrodynamic Design of Planing
    Hulls", a few pages prior to Figure 16 which he states that this is the case. Unfortunately, this paper was not the one that I was looking for as it only deals with
    a deadrise of up to 20 degrees. My recollection is that a 25-28 degree deadrise gets you close to 8 degrees.

    I am curious that you would seem surprised at this. The vertical lift forces becomes less due the weakening vertical lift vector due to deadrise so it is presumable that the optimum angle would not be fixed at your 4 degrees at any deadrise. Also at significant higher speeds, trimming up to reduce skin friction can increase boat speed reducing wetted surface. This paper is pretty thorough and Savitsky is recognized as an expert in his field. I was merely regurgitating information that I had read that was a hydrodynamic analysis of various deadrise hulls.

    Also, the optimum trim angle is speed dependent for a given deadrise 5-3963c946e5.jpg 5-3963c946e5.jpg

    The lower left hand diagram is the one that shows the relationship.
     
    Last edited: Nov 8, 2019
  10. Mr Efficiency
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    Mr Efficiency Senior Member

    The trim angle will take care of itself, at varying conditions of loading and speed, and sea conditions, what is "optimal" will vary, too little may slow the boat, too much may lead to less comfort, too little can also cause bow steer, or a wet ride, it is desirable to have a way to adjust the trim either way, to suit circumstances, and with trim tabs available, all the better.
     
  11. pacblue
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    pacblue Junior Member

    I related to you my real life experience outside of tank testing or theoretical calculations. The 4 degrees i have given you is a good number, pick up any magazine and you will see a typical range of 3 - 5 degrees, which works out to an average 4 degrees for an optimum trim underway. There are outliers of course but it has worked well for me, stern drive, outboards and inboards.

    I would not take the lower left graph to the bank as most boats have trim control to achieve their optimum, and very few manufacturers want their products dialed in at high trim angles.
     
  12. Mr Efficiency
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    Mr Efficiency Senior Member

    the tacho will tell you what optimum trim angle is, so far as resistance is concerned, though the utility of the boat may be affected.
     

  13. tom28571
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    tom28571 Senior Member

    Lots of boats run with a bow high and stern down attitude. Some on purpose and some due to poor design. Many Carolina Sporfishermen are constructed with a bit of upward sweep of the buttocks at the stern. The purpose is the same as you wish to do although they do it to hold the bow high in large waves so they can run at higher speed and make more money by decreasing time spent in getting to and from fishing grounds. They are more interested in getting to and from fishing grounds than in efficiency of the boat. Adding sternflaps mounted to the transom curved up instead of the normal down angle will do this and you don't need to mess up a boat's structure. Also allows you to experiment and either get to the right stern depression (bow lift) or make it adjustable.

    Normally we want to have absolutely straight buttocks near and at the stern for best trim and speed. I have "fixed" several boats by getting rid of unintended convex (up sweep) of stern buttocks, but different problems call for different solutions. Small changes in buttock lines can have large effect on a boat's attitude.
     
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